: 2005 Tire Size Question for Fronts and Rears



stevenriz
05-04-09, 09:06 AM
Hi everyone. Having purchased my first STS, I am learning more and more each day about these cars. The first thing I noticed when driving this 2005 was how well it handled. While getting gas, I looked down at the rear tire and read off the size. hm, 235 50 zr 18's and I'm like, cool that is a nice size tire... You probably know where this is going....

a couple days later. I was washing the car, and while washing the front wheel, checked out the size. hm, 255 45 zr 18's... Wait, the fronts are bigger then the rears! So I checked around the forum and it seems garages and some dealers do a tire rotation without knowling there are 2 different tire sizes on this car. Is that the general assumption here?

questions are...
- is it generally accepted that you can rotate tires like this? is it safe?
- do I gain or lose any performance with the rears on the front and vice versa?
- I am now concerned that when I rotate the tires back to where they belong, I will have worse cornering performance with the old fronts. Could this be the case?
** Most Imporant: Are the front and back wheel sizes the same or are the fronts smaller?
- If the wheel sizes are the same, could/would I get 245's all around? that way we can start performing the traditional radial tire rotation (front to back/back to front)....

thanks all...

Steve

bbshriver
05-04-09, 09:56 AM
Traditionally if you have staggered tire sizes "rotating" the tires is really just swapping them side to side... this doesn't work if they are directional though (unless you take the tire off the wheel and remount the other direction).
If your tires are backwards (wider in front) you need to put them back the way they are supposed to be. The only time I have heard of wider tires in front is on some of the Grand Prix GXP's but those are front wheel drive.

I don't know that having wider tires in the front is "unsafe" on an STS, but overall handling and performance should be better with the tires in their proper position, however if most of your driving is on public roads you probably never push hard enough to be at the limit anyway, I know I don't.

In any event, it's currently set up wrong and should be corrected.

z06bigbird
05-04-09, 10:09 AM
Life is just not that simple, but you have a fantastic car.

DTS_BOI
05-04-09, 02:19 PM
Great car! But switch the tires back to their proper position..... Immediately.

next2pool
05-04-09, 04:38 PM
I suspect a lot of these are running around with the tires on the wrong end. Wider tires (the 45's) will be worse for hydroplaning and can aggravate rut running (wandering in road grooves), and nibbling on ridges. It won't be huge, but the 50's will be better on the front.

next2pool
05-04-09, 04:42 PM
questions are...
** Most Imporant: Are the front and back wheel sizes the same or are the fronts smaller?
- If the wheel sizes are the same, could/would I get 245's all around? that way we can start performing the traditional radial tire rotation (front to back/back to front)....

thanks all...

Steve

I believe the rear wheels are 7.5 wide while the fronts are 7, but I'm not sure they are all that way. On vehicles that have different widths front to rear, the wheels will be stamped accordingly.

EChas3
05-04-09, 09:46 PM
There's a lot of us that have found the DPO rotated the staggered tires. On my 2006 STS4 1SG the 4 wheels are identical. And my DPO had our ignorant dealer rotate the tires 3 times (according to the VIN-check service record)! I guess the dealer goes by 'the customer is always right' but that doesn't build confidence. :)

When I replace the tires, I'm probably going with 255R45/18's all around. The staggered tires are nearly the same circumference. Some may disagree with my plan, but the car didn't have any problems with them reversed for about 30,000 miles. I will probably change to a softer/quieter tire.

Rotating tires does help; I am getting great tire wear. I've gotten better than rated wear many times by frequent tire rotation. With my STS, the fronts are just showing a bit of wear on the outside edges. The rears are very good. Previous Owner must have babied this ride! Unless the brakes were changed elsewhere, they're original, too. When the dealer restored the tires to their proper locations, he pronounced the pads good. I'm going to have them evaluated again at the next oil change. (I'd rather replace pads than all the rotors)!

The door-jamb label shows 235/ZR/50 & 255/ZR/45 18's. The tires themselves have the M+S 97W & 99W labelling, too. With the extra drag & weight from AWD it may not reach the limited max speed. I suppose I'd better go west to find a place where 150 MPH could possibly be argued to be 'safe & reasonable'!

stevenriz
05-05-09, 11:12 AM
I did notice when I drove the car off the lot that it followed the road a lot so I had the allignment checked and all the wheels rebalanced on the dealer's tab of course. Now I am worried when I rotate the tires back the way they should be if the alignment will be off. Should I warn the dealer that the tires were rotated when they shouldn't have? and that if the alignment is now off that I might need to have it realigned??

I haven't looked at the door jam but my tires are Z rated. 235/50 zr18 and 255/45 zr18... I love the handling and hope it isn't reduced by retating back.

Also, I'd like to rotate each side in one shot. Is there a spot under the car that I can jack it up so the entire side will be off the ground without bending or ruining anything underneath? Otherwise I will have to get out the spare and do one wheel at a time :(

steve

next2pool
05-05-09, 11:19 AM
""I did notice when I drove the car off the lot that it followed the road a lot so I had the allignment checked and all the wheels rebalanced on the dealer's tab of course. Now I am worried when I rotate the tires back the way they should be if the alignment will be off. Should I warn the dealer that the tires were rotated when they shouldn't have? and that if the alignment is now off that I might need to have it realigned??""


No--you'll be fine

pjbizjak
05-05-09, 01:14 PM
235/50's on the front and 255/45's on the rear. Rotate only side to side(non directional tires). You can jack up one side and use a jack stand then you can jack up the other side and swap tires at once, same for the front. Wheels should be 8" on the 18" front and rear. Air pressure should be 29lb cold.

EChas3
05-05-09, 07:22 PM
I agree that alignment shouldn't depend upon the wheel locations. A bent wheel would affect alignment but you'd notice that!

My tire info label specifies 30 PSI. The manual says see the label. Many STS owners post that they run a bit higher in front to save on tire shoulder wear. I've also seen a lot of posters recommend 4 PSI higher rear. Cadillac's recommendation is based on the best combination of ride & handling for most drivers. Personnally, I don't think there's no harm in trying various combinations (within reason) to get the feel you want.

My TPMS reads about 1 PSI low. If you don't have a high-quality and/or certified pressure guage, I'd recommend getting one. Some digital ones are good but the old pencil-sized ones are generally inconsistent junk.

As far as jacking points, the manual is quite specific on the points for use with the supplied jack. In their world, it's one corner at a time. With a good floor jack & stands there may be many solid jacking points. But it is a heavy car! I'd like to hear from someone who's done it. Don't take chances with this. If there's any doubt, you're betting your life!

turnerbend
05-05-09, 07:59 PM
With a good floor jack you can get both wheels off the ground using the front jacking position.

stevenriz
05-06-09, 08:43 AM
I have a good hydraulic jack in the garage. can I go under the control arm or be safe and use the spare tire jack?

bbshriver
05-06-09, 09:20 AM
My door jamb label also says 30lb, I have the 17" wheels not 18". I put mine at ~35, doesn't seem to ride too harshly.
30psi seems low to me for some reason? Tires are rated for 44psi "max" I tend to go 5-10psi below max depending on load.

next2pool
05-06-09, 05:12 PM
My door jamb label also says 30lb, I have the 17" wheels not 18". I put mine at ~35, doesn't seem to ride too harshly.
30psi seems low to me for some reason? Tires are rated for 44psi "max" I tend to go 5-10psi below max depending on load.


I just put new Kuhmos on and am running 32 psi. Other than ride, you need to keep track of the wear pattern since these wide aspect ratio tires may wear prematurely in the center, but you'll have plenty of time to keep track of that. Most people tend to wear the edges first so higher pressure is probably Ok.

next2pool
05-06-09, 05:36 PM
Another thing to think about is the weight capacity of tires not only at various pressures, but by size. The 255/45's will have more capacity than 235/50's at any given pressure so that's probably a good reason to keep the right tires on the front and rear if they are different. Right now I can't find the load & pressure chart on the Michelin website (all brands are the same chart) but it's a good idea to jack up the pressures a bit if carrying more load. The max tire capacities are at a max pressure of 44psi and go down from there.